1. Field of the Invention
This invention relates to power transmission belts and, more particularly, to a power transmission belt having a reinforcing canvas layer attached to a surface thereof.
2. Background Art
It is known to provide a reinforcing canvas layer on an exposed surface of a power transmission belt, such as a V-ribbed belt, low-edge V-belt, wrapped V-belt, etc. A rubberized canvas layer is commonly placed on the outside surface of a power transmission belt to add peripheral reinforcement thereto. The ends of the canvas layer are joined together by sewing. Typically, the ends are overlapped so that a step is defined on the outer surface of the belt.
To avoid the creation of this "step", some manufacturers leave a gap between the ends of the canvas layer. As a result, a portion of the belt body underlying the canvas layer is exposed.
In U.S. Pat. No. 5,224,905, assigned to the assignee hereof, a power transmission belt is disclosed wherein the ends of a canvas layer are abutted, edge to edge. As described more fully in that patent, this arrangement avoids localized repositioning of the load carrying cords that occurs with the sequential buildup of components over a step in a conventionally constructed canvas layer, avoids variation in the drive characteristics for the belt as when the step in the canvas layer contacts a cooperating pulley, and minimizes noise generation between the belt and cooperating pulleys, particularly at the step. While this construction permits smooth belt running and constant power transmission characteristics, it has one principal drawback.
More particularly, a problem arises in a system in which the belt is run in a reverse-bend mode. In the reverse-bend mode, the outside surface of the belt directly contacts at least one pulley in the system. For example, in serpentine drive systems on automobile engines, the outside surface of a V-ribbed belt encounters a flat pulley to effect driving thereof. While there is no step, as in the prior art, to contact a pulley and generate noise, there is still noise generated as the thread, which connects the ends of the canvas layer and protrudes outwardly of the canvas layer, encounters the pulley(s).
It is known to join the canvas layer ends using cotton thread, such as No. 40 cotton thread, to minimize noise generation as contact occurs between the thread and the system pulleys. This is commonly accomplished using known overlock machine-sewing techniques so that both the needle thread (that which is exposed on the outside surface of the canvas layer) and bobbin thread (that which is exposed on the inside surface of the canvas layer) are cotton. However, cotton thread requires twisting and thus this thread is relatively expensive as compared to mono-filament threads made from synthetic fibers having the same strength and resistance to cutting. Furthermore, cotton threads of relatively large diameter bind significantly as the threads are drawn through rubberized canvas. As a result, the sewing step is lengthened compared to that for threads of lesser diameter.
To overcome this last problem, it is known to use mono-filament threads of synthetic fiber. These threads have good strength, i.e. resistance to cutting, and are less expensive than cotton threads. Further, the mono-filament synthetic fiber threads can be drawn through the canvas layer with less resistance than cotton threads of equal strength. As a result, the sewing time is reduced. A high integrity, machine-sewn joint can thus be obtained a relatively low cost. However, the mono-filament threads made from synthetic fibers likewise have drawbacks.
The mono-filament threads of synthetic fiber are relatively inflexible compared with twines of natural fiber having the same strength and resistance to cutting and multi-filament twines of synthetic fibers spun by twisting a plurality of filaments together. When a belt using these threads is reverse bent, the canvas layer compresses along the length of the belt. The thread on the outside of the canvas layer which, in normal operation, is pressed into the canvas layer, bows outwardly and protrudes from the outer surface of the canvas layer. In so doing, it generates noise as it contacts the cooperating pulley.